>How does the chosen come back?
Certain souls can reincarnate all across timeline from body to body. The goal is to maximize the goodness of the entire timeline by spreading as much positivity and improvement as possible. Given that there is probably a limited number of souls that can do this (and are allowed to) only specific bodies are chosen for this mission in order to amplify the effectiveness of the timeline changes in a given set of subsequent timeframes.
At least that is my current understanding of it with some hunches.